mruby: Wrap flags-operation of scan_info in ruby method

Not just wrap C-API but set/unset each flag.
It is harmful to treats all flags at once like grn_scan_info_set_flags().
It is OK for low-level API, but I think it is good time to introduce more safety method.

If you don't think so, please feel free to say. I will rewrite.

TODO: It might be good to add <code>reset_all_flags</code>, <code>clear_flags</code> or something.